Create a Map<String, Integer> called carCountMap
Loop through all the keys of the first Map, the one with the owner/car String ArrayList
get the associated ArrayList of Strings
Loop through this ArrayList, getting each car String.
Check if car String is held by carCountMap. If so,
get the associated Integer, increase it by one
put it back into the carCountMap with the car String as key.
if not, then put a new entry into the carCountMap with the car name as key and 1 as the value.
end Loop through ArrayList.
end for loop